Talk:ITI White Paper: Publish/Subscribe Infrastructure

Conference Call February 12, 2008

After introduction of the original outline, the group consensus was that the white paper should closely follow the format of a profile, and, as originally proposed, concentrate on resolving XDS-specific problems, rather than stating general principles of publish/subscribe.

Use cases

Use cases are central to the development of the white paper, and the discussion produced several use cases for a general XDS workflow:

Unexpected notification

A patient in the emergency department has all her relevant available documents retrieved via XDS-b transactions. As initial triage of the patient is done, an additional document regarding diagnostic results for this patient is registered in the XDS registry. Currently, there is no way for the Emergency department to learn about the existence of this new information. With a publish/subscribe infrastructure, the initial query to the registry would be accompanied with a subscription request, as a result of which a notification (or the document itself) would be send to the emergency department. The subscription will be terminated once the patient is no longer under the care of the emergency department's institution.

Long-term subscription

A patient visits their PCP after being discharged from a hospital, which belongs to the same XDS affinity domain as the provider's organization. The provider sends a query to the affinity domain registry, and retrieves the hospital discharge summary. The patient also has follow-up visits with a specialist at the hospital, and these visit summaries (including diagnostic test results) are registered in the affinity domain registry. Currently, the PCP would have to periodically query the registry for documents about the patient in order to retrieve the follow-up visit summaries. With a publish/subscribe infrastructure, the PCP would have a subscription for all his patients, so that notifications (or the documents themselves) would have been received as the summaries were registered in the affinity domain registry.

Original Outline

Introduction

Event driven information exchange patterns dominate the data interchange in most healthcare settings. For example, most HL7 version 2.x interfaces send messages based on events within the sender's system. Most current IHE profiles assume either static, out-of-band determination of the senders and receivers of event driven information exchange, or describe query-response patterns. There is a need for a profiled dynamic infrastructure for event-driven information exchange patterns within IHE. This white paper describes such a framework based on the publish/subscribe data exchange model, and the methodology to apply the framework to IHE profiles.

Publish/subscribe patterns of data exchange are conceptually simple, and involve a limited numbers of actors and transactions. The transaction allow for automating the determination of information consumers based on events or content "topics". For example, if an IHE profile describes information content where a diagnosis is present and coded using a particular coding system, a subscriber can request to receive any transactions where one from a particular set of diagnosis codes is present. If the subscription is accepted, the system which keeps track of information recipients will start sending the transactions which match the described criteria to the subscriber.

The above example demonstrates two important issues which need to be addressed by profilers and implementers of publish/subscribe interactions. The first one is that the implementation of transactions and actors are dependent on the information exchange environment. Prescribing a specific technology for publish/subscribe is not a practical option - it is unlikely that, for example, IHE actors which exchange information using DICOM transaction will be inclined to add Web Services based transactions for enabling publish/subscribe. To address this issue this paper describes the publish/subscribe actors and their transactions in an abstract manner, and present the way to make these abstract constructs into regular IHE actors and transactions.

The second issue that needs to be addressed is the use of subscription topics. Based on the information exchange environment, topics can be described in various ways. This paper presents various approaches to specifying topics, and some discussion on how these approaches can be used in IHE profiles.

The implementation of publish/subscribe in healthcare environments also needs to take into account the need for security and privacy of the exchanged information. The methodology to include publish/subscribe transactions in IHE profiles contains a discussion on the security implications arising in such situations.

As a proof of concept, Appendix A of the paper demonstrates the use of publish/subscribe as part of the XDS profiles.